Major Challenges and Industrial Prospects of Sodium Battery Membranes

First Author: Xue Zhixin

Corresponding Author: Xu Guiyin

Affiliation: Donghua University

Summary of Results

The low cost, low temperature, and fast charging performance of sodium-ion batteries (SIBs) have attracted widespread attention. The membrane is one of the essential key components of the battery, affecting the interface structure and internal resistance of the battery, as well as characteristics such as capacity, cycling, and safety performance. Therefore, the membrane must have excellent wettability, mechanical properties, pore size distribution, ionic conductivity, and thermal stability. Currently, research on sodium-ion battery membranes mainly focuses on the modification of commercial PP and PE membranes and other new types of membranes, which have excellent chemical stability and cycling stability. These membranes have good electrochemical stability and high mechanical strength, but their stability at high temperatures is relatively poor, and there is a problem with poor electrolyte wettability. Therefore, in sodium-ion batteries, the membrane is one of the challenges, and there are still significant challenges in practical applications.

Introduction

Lithium-ion batteries and sodium-ion batteries emerged in the late 1970s. Lithium-ion batteries have developed rapidly due to their electrochemical performance and currently dominate the market. Lithium-ion batteries occupy a dominant market share in many application scenarios, such as smartphones, electric vehicles, solar energy storage, and aerospace-related manufacturing. Meanwhile, the export proportion of lithium batteries is also increasing year by year (Figure 1a-b). However, this widespread use has led to a shortage of lithium, and costs have increased accordingly. Statistics show that global lithium resources are mainly concentrated in South America and Australia, while China’s lithium resources account for only 6% of global reserves (Figure 1c). The shortage of lithium will restrict its sustainable development.

Sodium has similar physical and chemical properties to lithium, but its abundance on Earth is 440 times that of lithium (Figure 1d). Sodium resources are widely distributed and easily extracted, and compared to lithium batteries, the cost of large-scale commercialization of sodium batteries is significantly lower. In recent years, with the development of the energy vehicle industry in our country and the increase in market demand, the research and development of SIBs have also made steady progress. SIBs outperform lithium batteries in low-temperature performance, fast charging performance, and safety performance, but still have some shortcomings. The low energy density, low cycling stability, and rapid capacity decay of SIBs limit their potential applications.

The structure of sodium-ion batteries is similar to that of lithium-ion batteries, mainly consisting of key materials such as positive electrode materials, negative electrode materials, electrolytes, and membranes. Sodium ions oscillate back and forth between the positive and negative electrodes, similar to a rocking chair. During charging, sodium ions are extracted from the positive electrode, pass through the electrolyte and membrane, and eventually embed in the negative electrode. Electrons flow through the external circuit from the negative electrode to the positive electrode to balance the charge, and the discharging process is the opposite of this (Figure 2). As an important component of the internal structure, the membrane can isolate the positive and negative electrodes, store the electrolyte, and facilitate the free transport of sodium ions, significantly impacting the electrochemical performance of the battery. The ideal membrane for high-energy-density sodium batteries needs to possess excellent chemical stability to have high corrosion resistance to organic electrolytes; high mechanical strength to prevent sodium dendrites from penetrating the membrane, causing internal short circuits; high wettability to expand the contact area between the membrane and the electrolyte, thereby improving ionic conductivity, enhancing the battery’s charge and discharge performance and capacity; appropriate pore size to promote the formation of uniform sodium ion channels, thus improving ion transport efficiency; good thermal stability to maintain the integrity of the membrane, continue to isolate the positive and negative electrodes, and prevent short circuits; thin membranes can provide more space for the active materials of the positive and negative electrodes, thus improving the energy density of the battery; low production costs allow sodium-ion batteries to be widely promoted and sustainably developed in the market.

Major Challenges and Industrial Prospects of Sodium Battery Membranes

Figure 1: (a-b) Global lithium battery market size and China’s lithium battery export volume; (c) Global lithium resource distribution; (d) Comparison of sodium and lithium properties.

Figure 2. Structure of sodium-ion batteries (left) and performance requirements of membranes (right).

Currently, the main materials for sodium-ion battery membranes are polyethylene, polypropylene membranes, and their composite membranes. Polyolefin membranes are low-cost, have strong mechanical properties, and good chemical stability, but their thermal stability and electrolyte compatibility still have issues. Glass fiber has high porosity, excellent thermal stability, and high ionic conductivity, and is widely used as a membrane for sodium-ion batteries in laboratories, but still has problems such as large thickness and poor mechanical properties (Figure 3). To avoid affecting the electrochemical performance of sodium batteries while having high thermal stability, researchers have improved the thermal stability of the membranes by coating them with inorganic ceramic powders or polar polymers with good thermal stability, thereby effectively enhancing the battery’s safety, high-power fast charging, and discharging performance. However, this may lead to a reduction in pore size and an increase in membrane thickness, negatively impacting battery performance. Additionally, cellulose-based materials are widely available, have high wettability, good thermal stability, biocompatibility, and environmental friendliness, meeting the needs for sustainable development in the battery field. However, cellulose-based membranes are still in the laboratory research stage and require further exploration for commercialization. Furthermore, using interlayers that promote interfacial ionic transport, designing composite membranes that prevent sodium dendrite growth, or using inorganic solid electrolytes with low elastic modulus are also feasible.

Figure 3. Comparison of thickness, porosity, wettability, ionic conductivity, and mechanical properties of a) PP, b) PE, c) PI, and d) GF.

In recent years, the number of companies involved in the sodium battery industry has rapidly increased. By the end of 2022, the sodium battery industrialization had initially formed an industrial chain, and in 2023, large-scale applications were achieved. However, although the technical routes for positive and negative electrode materials have matured, the development of membranes is still in the laboratory research stage. Compared to lithium-ion batteries, sodium batteries require more research technologies for membranes. Overall, current and future research on sodium batteries will focus more on membranes. Producing low-cost, high-safety sodium battery membranes is the trend of future market demand.

Conclusion and Outlook

The rapid development of the new energy market and the high cost of lithium batteries have accelerated the industrialization of sodium-ion batteries. Due to the requirements for high energy density and safety, there is still a large space for the development of SIBs. Currently, their positive and negative electrodes and electrolytes have been industrialized, but the research and industrialization of membranes are still lagging. As one of the key components of sodium-ion batteries, membranes determine the electrochemical performance and safety performance of the batteries. In-depth research on new materials, structural design optimization, and functional membranes aims to manufacture more efficient, stable, and safe sodium-ion batteries. Due to the poor compatibility of polyolefin membranes with sodium battery electrolyte systems, as well as their poor wettability and thermal stability, simple modifications have been made to polyolefin membranes through coatings and grafting polar groups to make them suitable for sodium-ion battery systems. In addition, some new membranes such as polyether ether ketone and polyphenylene sulfide can not only improve thermal stability but also enhance electrolyte wettability and ionic conductivity. However, sodium dendrites severely affect the safety performance of the batteries, and the interface issues between the membrane and the positive and negative electrodes require more detailed research.

Original link:

https://www.sciencedirect.com/science/article/abs/pii/S1748013224000306

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